Hydrocarbons are often produced from well bores by reciprocating downhole pumps that are driven from the surface by pumping units. A pumping unit is connected to its downhole pump by a rod string, which is assembled from a number of sucker rods. Often, the downhole pump is referred to as a sucker rod pump and the surface pumping unit that reciprocates the rod string is referred to as a pump jack, although other terms may also be used by routineers in the art. Several types of pumping units are known in the art, including the prolific walking beam style pumps, and pumps that employ a piston-cylinder arrangement.
The uppermost rod in the rod string is called a polished rod, so named because of its smooth finish. The pump jack carries the polished rod, typically via a wire rope bridal and carrier bar. The polished rod extends through a packing gland or stuffing box at the wellhead for providing a dynamic well seal as the polished rod strokes up and down. A rod string of sucker rods hangs from the polished rod within a tubing string located within the well casing. The rod string is connected to the plunger of the subsurface pump. In a reciprocating cycle of the pump jack, formation fluids flow into the well and pump housing during the downstroke, and well fluids are lifted within the tubing string during the rod string upstroke.
One type of pump jack is a wellhead-mounted linear lift system. The linear lift system has an elongate frame vertically oriented and mounted atop the wellhead. The frame may be supported with guy wires. The top of the frame carries a ram assembly oriented to be in line with the wellhead. The ram assembly may be hydraulic or pneumatic, for example. A piston rod extends downwardly from the ram assembly and connects to the polished rod of the sucker rod pump above the wellhead. Application of a pressurized fluid to the ram assembly lifts the piston rod, the polished rod, the rod string, and the plunger of the downhole pump to lift well fluids within the tubing string. A subsequent release of pressure to the ram assembly allows the piston rod, the polished rod, the rod string, and the plunger of the downhole pump to descend, thereby completing one pump cycle.
Such linear lift systems are typically characterized by a small footprint, low weight, high load capacity, an easily adjustable ultra-long stroke, and a simple, low-maintenance actuator that uses a single-acting ram. Because of these advantages, there are circumstances in which it is desirable to use a wellhead-mounted linear lift system.